1. Field of the Invention
This invention relates to azoalkane mixtures containing unsymmetrical azoalkanes, a process for preparing such mixtures and the use of such mixtures in polymerizing vinyl monomers and curing unsaturated polyester resins.
2. The Problem
Various important properties of azoalkanes (R--N.dbd.N--R'), such as physical state, solubility, volatility, toxicity, thermal stability, and initiator efficiency, are dependent upon the nature of the alkyl groups R and R'. The thermal stability of the unsymmetrical azoalkanes (R.noteq.R') is intermediate between the stabilities of the symmetrical azoalkanes R--N.dbd.N--R and R'--N.dbd.N--R' (P. S. Engel, Chemical Reviews, 80, pp. 101-109 (1980). Therefore, many of the unsymmetrical azoalkanes have unique thermal and initiator properties (depending upon R and R'), which cannot be duplicated by the symmetrical azoalkanes. These unique properties make them commercially desirable as free radical initiators. This is especially true when R and R' are tertiary groups.
Mixtures of an unsymmetrical azoalkane and the corresponding symmetrical azoalkanes have unique thermal and initiator properties that may extend over a wide temperature range depending upon the R and R' groups. For example, when tert-octylamine (1,1,3,3-tetramethylbutylamine) and tert-butylamine are used in the sulfamide reaction, a mixture of di-tert-octyldiazine (which has a ten hour half life (t-1/2) of 107.degree. C.) and tert butyl tert-octyldiazene (which has a 10 hour half life of 137.degree. C.) is obtained; the 10 hour half lives were extrapolated from data in J. W. Timberlake et al 1976 publication in the Journal of Organic Chemistry, vol 41, page 1666. Di-tert-butyldiazene (which has a 10 hour t-1/2 about 160.degree. C. and a b.p. of 109.degree. C.) is removed during the workup. Mixtures of tert-octylamine with tert-amylamine or tert-dodecylamine (Primene 81-R product) give mixtures of azoalkanes with similar half lives as above. Mixtures of initiators of various activities are commonly used in various polymerization processes. Mixtures of peroxides of different half lives are used in the polymerization of styrene, vinyl chloride, and ethylene. Mixtures of azonitriles are also used in polymerizations but these leave toxic residues in the polymers. Mixtures of azoalkanes derived from tert-octylamine and another tertiary alkyl primary amine are particularly useful where a low or intermediate temperature initiator is required to initiate the polymerization at a reasonable rate and a higher temperature initiator is required to finish the reaction. This is especially true in the bulk polymerization of styrene where a high temperature initiator is required to obtain faster conversion to polystyrene. Peroxides cannot be used in this capacity because they degrade polystyrene at the higher temperatures. Azonitriles are too unstable to be used as finishing catalysts. In fact, prior to our invention, no such suitable commercially feasible initiator system existed.
The ratio of the azoalkanes in the mixture and therefore the activity of the azoalkane mixture can be varied by varying the ratio of the amines used in the sulfamide reaction. The activity of the mixture can also be altered by adding one of the pure symmetrical diazenes to the mixture.
Prior to the present invention, mixtures of azoalkanes containing unsymmetrical azoalkanes were not commercially available due to the difficulty involved in preparing the unsymmetrical azoalkanes. Prior to the present invention, unsymmetrical azoalkanes were prepared by a variety of methods which were tedious, had many steps, and were not applicable to all classes of unsymmetrical azoalkanes.
3. Prior Art
(A) Unsymmetrical Azoalkanes
P. S. Engle and D. J. Bishop prepared a series of unsymmetrical sulfamides by reacting methyl, isopropyl and tert-butyl sulfamyl chlorides with 2-amino-2-methyl-3-butene. The unsymmetrical sulfamides were oxidized to the corresponding unsaturated azoalkanes with basic bleach (P. S. Engel and D. J. Bishop, J. Am. Chem. Soc. 97, 6754 (1975). The sulfamyl chlorides are difficult to prepare and were made by the method of Weiss and Schulze (G. Weiss and G. Schulze, Justus Liebigs Ann. Chem., 729, 40, 1969). The method involves direct treatment of an amine hydrochloride with sulfuryl chloride either with or without a Lewis acid catalyst. The method is limited to simple alkyl amines and is often characterized by long reaction times, large excesses of reagents and low yields (J. A. Kloek and K. L. Leschinsky, J. Org. Chem. 41, 4028 (1976). Tert-Butyl sulfamyl chloride was especially difficult to prepare. Kloek and Leschinsky developed a better method of preparing sulfamyl chlorides but they still only got a 34% yield of tert-butyl sulfamyl chloride (J. A. Kloek and K. L. Leschinsky, J. Org. Chem., 41, 4028 (1976).
Treatment of tertiary alcohols with chlorosulfonyl isocyanate gives monoalkyl sulfamyl chlorides (J. B. Hendrickson and J. Joffe, J. Am Chem. Soc., 95, 4084, 1973). Timberlake used this method to synthesize N-tert-butyl-N'-tert-octylsulfamide in 44% yield (J. W. Timberlake, M. L. Hodges and A. W. Garner, Tetrahedron Lett., 3843, 1973 and J. W. Timberlake, J. Alender, A. W. Garner, M. L. Hodges, C. Ozmeral and S. Szilagyi, J. Org. Chem., 46, 20282, 1981). The sulfamide was oxidized with tert-butyl hypochlorite and sodim hydride to tert-butyl tert-octyldiazene. N-tert-Butyl-N'-(adamant-l-yl) sulfamide was prepared in 45% yield by the same route. There is no evidence it was oxidized to the azo. Apparently, tert-butyl tert-2,2,4,6,6-pentamethyl-4-heptyldiazene was also prepared by this route (J. W. Timberlake and A. W. Garner, J. Org. Chem., 41, 1666, 1976).
R. Ohme and H. Presuschhof tried preparing the unsymmetrical propyl butyldiazene by oxidizing a mixture of N,N'-di-propylsulfamide and N,N'-di-butylsulfamide. They only obtained 1,1'-azopropane and 1,1'-azobutane. None of the unsymmetrical azoalkane was formed (R. Ohme and H. Preuschhof, Liebigs Ann. Chem., 713, 74, 1968). This is the only example found in the literature of the direct preparation of a mixture of azoalkanes.
R. E. MacLeay and C. S. Sheppard (in U.S. Pat. No. 4,007,165) prepared unsymmetrical azoalkanes by reacting tert-alkyl-.alpha.-chloroazoalkanes with Grignard reagents and phenyl or alkyl lithium reagents. Although the unsymmetrical azoalkanes prepared by this route proved to be very good free radical initiators, the process proved to be commercially unattractive. Unsymmetrical tert-octyldiazenes were disclosed in this patent.
(B) Mixtures
Mixtures of azoalkanes of the structures R--N.dbd.N--R, R--N.dbd.N--R' and R'--N.dbd.N--R' and mixtures of R--N.dbd.N--R' and R'--N.dbd.N--R' were not found in the prior art.
(C) Process
The process of making mixtures of azoalkanes where at least one component of the mixture is an unsymmetrical azoalkane (R--N.dbd.N--R') was not found in the prior art.
Processes are known for the oxidation of N,N'-dialkylsulfamides to azoalkanes. Ohme and Preuschhof oxidized a mixture of N,N'-di-propylsulfamide and N,N'-di-n-butysulfamide to a mixture of 1,1'-azopropane and 1,1'-azobutane with basic bleach (R. Ohme and H. Preuschhof, Liebigs Ann. Chem., 713, 74, 1968). Ohme and Schmitz developed a general synthetic method for the preparation of low molecular weight azoalkanes (R. Ohme and E. Schmitz, Angew. Chem. Int. Ed. Engl., 4, 433, 1965). They found that the dialkylamides of sulfuric acid in a solution of alkali react with 2 moles of NaOCl at 20.degree.-60.degree. C. to form aliphatic azo compounds. They prepared azopropane, azobutane and azocyclohexane in this manner.
J. C. Stowell prepared 2,2'-azoisobutane in 84% yield by oxidizing N,N'-di-tert-butylsulfamide with basic bleach for 3 hours in pentane (J. C. Stowell, J. Org. Chem., 32, 2360, 1967). Ohme and Preuschhof oxidized the same sulfamide at 60.degree. C. with basic bleach and also obtained an 84% yield of 2,2'-azoisobutane.
However, according to the prior art it is very difficult to oxidize many of the higher molecular weight N,N'-dialkylsulfamides with basic bleach. In 1972 J. W. Timberlake and co-workers attempted to prepare di-tert-octyldiazene and di-tert-heptyldiazene by this route but were unsuccessful (J. W. Timberlake, M. L. Hodges and K. Betterton, Synthesis 1972, 632-34). Treatment of either N,N'-bis(2,4,4-trimethyl-2-pentyl)sulfamide (i.e. N,N'-di-tert-octylsulfamide) or N,N'-bis-2,3,3-trimethyl-2-butyl)sulfamide (i.e. N,N;-di-tert-heptylsulfamide) under the conditions specified in Ohme's articles gave no azo compound. Timberlake recognized that the oxidation of sulfamides to azo compounds with basic bleach was not adaptable to all azos. The conditions were too vigorous for isolating unstable azo compounds and solubility problems in several cases led to quantitative return of starting sulfamides. Therefore, Timberlake developed a more complex method of converting N,N'di-tert-octylsulfamide into di-tert-octyldiazene using sodium hydride and tert-butyl hypochlorite (J. W. Timberlake, M. L. Hodges and K. Betterton, Synthesis, 1972, 632-34).
In 1978 a Japanese patent described a process for oxidizing N,N'-di-tert-octylsulfamide with bleach and caustic in the presence of a phase transfer catalyst (Japan Kokai 77,128,305; C. A., 88, 120602m, 1978). The reaction was run at 40.degree. C. and required 10 hours to complete. This was the first indication that di-tert-octyldiazene could be prepared by the aqueous bleach route. The process required a phase transfer catalyst and a long reaction period.
In early 1982 a European Patent was published describing a photopolymerization process (R. Ohme and E. Schmitz, Angew. Chem. Int. Ed. Engl., 4, 433, 1965). In the patent, there is a description of the preparation of 2,2'-azobis(2,4,4-trimethylpentane) which is also referred to as di-tert-octyldiazene. The di-tert-octyldiazene was prepared by oxidizing N,N'-di-tert-octylsulfamide with bleach and caustic solution for 20 hours at 35.degree. C. The yield was only 50% after purification by vacuum distillation.
In 1971 A. Ohno and co-workers reported the preparation of azobis(2-propyl)-2-propane by sodium hypochlorite oxidation of the corresponding sulfamide (A. Ohno, N. Kito and Y. Ohnishi, Bull. Chem. Soc. Japan, 1971, 44, 470-474). The azo was obtained in 38% yield after stirring 7.0 grams of the sulfamide in 50 mls of hexane with 150 mls of 10% NaOCl for 35 hours at room temperature.